1. Field of the Invention
The present invention relates to a method for controlling a speed at which a wound tape is transported and a tape unit equipped with the control method, and more particularly to a method for controlling a transport speed of a tape wound around a pair of reels for use in a video tape recorder (hereinafter referred to as a VTR), a cassette deck or the like and a tape unit equipped with the control method.
2. Description of the Related Art
A magnetic tape for use in a VTR or the like is arranged such that its both ends are fixed to a supply reel and a take-up reel, respectively, and its middle portion serves as a magnetized surface which can be magnetized in accordance with a recording signal. The magnetized surface contacts to a magnetic head disposed between the supply reel and the take-up reel so that the magnetic head reads information recorded on the magnetic tape, and transfers the information to predetermined electric circuits.
When a reel hub of the supply reel or a reel hub of the take-up reel is driven by a capstan motor, the supply reel and the take-up reel are rotated in the same direction. Therefore, the magnetic tape is transported while contacting to the magnetic head. An operation in which the tape is unreeled from the supply reel and taken up by the take-up reel is called a feeding operation (F) while an operation in which the tape is rewound from the take-up reel side to the supply reel side is called a reverse (REV) operation.
When reproduction or recording is carried out, the magnetic tape is transported at a normal speed. But in order for a user to find a desired recording position of the magnetic tape and bring the position in front of the magnetic tape swiftly, there are provided functions of a fast forward (FF) and a rewinding (REW) in which the magnetic tape is transported at a high speed.
In a recently developed VTR, in order to reduce a waiting time of the user, a microcomputer is introduced for controlling the revolution rate of the capstan motor which is utilized for transporting the magnetic tape at a high rate in the fast forward (FF) and the rewinding (REW) modes.
When the magnetic tape is transported at a high speed, how to stop the tape must be considered. For example, when the rewinding mode is operated, the magnetic tape is rewound up to its terminal end. Therefore, if speed reduction starts too late near the terminal end, the terminal end of the magnetic tape passes over the magnetic head and a leader tape can be exposed. In the worst case, the reel is still rotated after the leader tape has been completely rewound. As a result, the magnetic tape and the leader tape can be cut or split at a bonding portion between the magnetic tape and the leader tape or at a bonding portion of the leader tape and the reel hub.
Nowadays, as a method for halting the magnetic tape transported at a high speed, there is proposed a method in which a ratio of the radii of the supply reel and the take-up reel around which the magnetic tape is wound is checked so that an amount of magnetic tape wound around each of the supply reel and the take-up reel is determined. Specifically, the magnetic tape is transported at a high speed until the radius of the magnetic tape around the supply reel becomes equal to the radius of the magnetic tape around the take-up reel, and thereafter the speed is reduced to a speed which permits the tape to be stopped quickly, whereby the magnetic tape is protected from being damaged.
Further, if a relatively short magnetic tape is to be controlled in its transportation speed in the fast forward or the rewinding modes, there has been known a technique as follows. That is, after the magnetic tape is loaded, when the fast forward or the rewinding is started, a pinch roller is pressed against the magnetic tape to measure an area of the magnetic tape, i.e., a radius of the roll of the magnetic tape, and then a length of the magnetic tape is determined on the basis of the measured result, and the magnetic tape is controlled at its transportation speed based on the length thus determined.
However, the method for controlling the speed of the magnetic tape by checking the ratio of radii of the tape rolls on the supply reel and the take-up reel involves the following drawbacks. Specifically, if a tape transported at a high speed is a magnetic tape with an extremely short length such as a tape for use in a compact cassette tape (hereinafter referred to as a C cassette) of a video home system (hereinafter referred to as a VHS) or a cut special magnetic tape, for example, the timing for effecting the speed reduction in accordance with the check of the ratio of radii of the tape rolls to the supply reel and the take-up reel, tends to be late, causing damage to the tape.
Further, the method in which a pinch roller is pressed against the magnetic tape to control the speed of the tape involves the following drawbacks. That is, if the magnetic tape is set to carry out the FF/REW mode under condition that information concerning the length of the magnetic tape is not available, then the pinch roller should be pressed against the magnetic tape for measuring the length of the magnetic tape prior to setting the high-speed transportation mode. For this reason, it is necessary to carry out an operation for pressing the pinch roller against the magnetic tape, which fact causes problems of a loss time upon starting the FF/REW operation and a sense of incongruity in operation.
Further, it is unavoidable for the microcomputer to require a finite calculation time and also it is necessary for a read only memory (hereinafter referred to as ROM) to have a large capacity for calculating the cross-sectional area of the magnetic tape roll, storing a table for memorizing the measured result, calculating the remaining amount of the magnetic tape and so on. Therefore, the control method itself involves an essential problem.